Potassium metaphosphate, a.k.a. potassium Kurrol's salt, is known to be practically insoluble in water. However, potassium metaphosphate can form a solution in the presence of other metal cations such as sodium cations (Comprehensive Inorganic Chemistry, Vol. 2, chapter 20, page 524-1973). Potassium metaphosphate shows the unusual capability of providing a high viscosity material when the molar ratio of K/P ranges from about 0.8 to 0.99 with a maximum at about 0.9 (Phosphorus and its Compounds, Vol. I by J. P. Van Wazer, 1961, pages 676-677).
Potassium metaphosphate can be prepared by reacting potassium salts, such as potassium chloride and potassium hydroxide, with phosphoric acid.
In the past, various problems have arisen in preparing potassium metaphosphate by heating phosphoric acid with potassium hydroxide at a K/P molar ratio of 0.9-0.95 at temperatures above 500.degree. C. In order to prepare potassium metaphosphate of high voscisity, high temperatures are needed. These temperatures stress the effectiveness of stainless steel at avoiding corrosion to its limits. High temperatures also cause the potassium metaphosphate to melt. Corrosion of the reaction vessel occurs at points where the melt contacts the stainless steel. Potassium metaphosphate so prepared often had a slight green discoloration due to the corrosion. While it is easiler to manufacture potassium metaphosphate at lower temperatures which do not present the corrosion problems, viscosity suffers because of the decreased temperature.
Potassium metaphosphate has also been prepared by contacting phosphoric acid with potassium hydroxide in a molar ratio of 1:1.10 (K/P molar ratio of 0.909) and agitated to produce a monopotassium phosphate liquor. The pH (of a 1% solution) was adjusted to pH 3.5-3.6 and Baume' of 28.degree.-30.degree.. The monopotassium phosphate was heated in a rotary kiln at 580.degree. C. to 620.degree. C.